It is only needed when it is needed. I use it on about half of my lenses. When I get a new lens, I test it with "real life" shots, and carefully scrutinize them for focus accuracy. If I see nothing wrong, I do not do MA. It does not stop there, I may find problems later, and then do MA.

I do it manually, with a target at my normal shooting distance with that lens. With zooms, I do it at the longest FL.

Yeah I recommend it for any lens that has a wide aperture. If anything you can see how consistent the AF is. The software is automated and it does repeated tests and then averages it out. What's interesting is how the focus at these wide apertures varies - click +10, click again +6 and so on. So that means at any given time, even with a calibrated lens you're not gonna be hitting the target with 100% accuracy every time. All your doing is increasing that probability or getting it closer to accurate. Some lenses may only be slightly off in which case you might get away with leaving it at zero. But some will clearly be off when you see Reikan in action.

Forgot to mention it also depends a lot on your typical shooting distance. The further the subject is from the lens the more likely it is to be in sharp focus as the dof increases. So calibrate the lens at your working distance.

+100 X 1I have seen enough obsession here. No doubt its a very useful tool when you NEED it.There are plenty of conflicting views here. Opinion range from: 1) You should get every lens you have and put it through test charts at every focal length, distance, with temperature changes of +/- 1 deg in bright light2) If you see a problem, try AF microadjust with real life things you tend to shoot most. I will leave it up to you to decide what you want to believe. Must add that most respectable pro's (any yes Ken R is not included here - neither respectable nor pro) tend to suggest #2. I am adding an article from what should be most relevant source - The Canon itself:

There are many people aggressively pushing for FoCal (not suggesting any financial interest; it may be their obsession). If you believe in nice and valid argument that $60 is less than 1% of $6000 or more you might have spent on gear, you can stop here. FoCal has been widely reported to work well. IMHO, it might be more damaging to your time than your wallet. If you are one who can't waste $1 even if you can afford everything you want, there are free ways. One from Arash Hazeghi looks most interesting (would love to hear opinions of people who have used it). Google or see description in link below.

2) If you see a problem, try AF microadjust with real life things you tend to shoot most.

I think that's a reasonable approach. The only possible downside is that if a lens-body combo is a little off, the images will still look pretty sharp - it's just that they could be a little sharper...but how would you know that?

One 'quick-and-dirty' test is to compare contrast-detect (live view) AF with phase-detect (viewfinder) AF. Set up on a tripod, take a few shots in Live View, then a few shots with standard AF. If there's a noticeable difference in sharpness between the sets of images with the Live View images winning, AFMA would be a good next step.

When I put my 135L on my 5DII, shots looked soft. It needed an AFMA of -11, after that it was very sharp. That same lens on my 1D X needs no adjustment. I didn't get around to running an AFMA test on my 600/4 for a while after getting the lens, but the images appeared sharp. Sure enough, it tested out at +1 for the bare lens, and -1 with the 1.4xIII - not really significant for an f/4 lens.

I check all my new lenses as soon as I get them. The reason is simple, if there is a problem, its easiest to find it by using a tool like Focal.

It has nothing to do with obsession, a properly functioning lens is important, and it can be returned and replaced if you check it out immediately. I once bought a used lens, it seemed fine, and after a few weeks, I used it to take photos of my 90 year old father at Christmas. Every one of them was coming out oof. Finally, I spotted that by reviewing them at magnification on the camera LCD. The only other lens I had was a f/4, and in the poor light, I had to use extreme ISO. I lost a lot of photos that could not be replaced, he passed on without another Christmas.

Lesson learned - check new equipment.

How critical is the AFMA adjustment? Most Canon lenses are close enough to where it is not a issue, but there are enough exceptions to make it worth checking. Third party lenses tend to be a lot more variable.

When a lens requires more than about a 5 point adjustment, the improvement is noticeable, and 10 points or more is flat out obvious.

I find, not surprisingly, that used lenses tend to need AFMA more than new ones, but there are exceptions, about 1 in 4 had a adjustment of 8 pts or more.

One 'quick-and-dirty' test is to compare contrast-detect (live view) AF with phase-detect (viewfinder) AF. Set up on a tripod, take a few shots in Live View, then a few shots with standard AF. If there's a noticeable difference in sharpness between the sets of images with the Live View images winning, AFMA would be a good next step.

Couldn't agree more. This is my absolute must after I buy a new lens or try new combo.

I think its also relevant to add here, that for situations where DoF is super thin (and hence optimal AFMA most necessary) finding a contrasty signal for AF sensor to latch on (mostly in sub-optimal light) is most critical. Would definitely increase razor sharp keepers (provided everything else is in tolerable range).